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Solar Thermal FAQs

Frequently asked questions about solar thermal systems


Solar Thermal FAQs

I have a Combi boiler. Is solar thermal an option for me?

These are some of the issues to consider if you have a Combi boiler:

200L: 600mm diameter x 1500mm height

250L: 600mm diameter x 1750mm height

300L: 650mm diameter x 1750mm height

I don’t want to have stored hot water, but want to keep my Combi system’s hot water on demand. Are there any other solar thermal options?

It is possible to get a dedicated solar pre-heat tank that would feed pre-warmed water to the Combi boiler.  A clever Combi diverter valve can be used in this setup. The boiler needs to have a minimum temperature difference to heat the water. If the water is hot enough the valve diverts the hot water to bypass the boiler. If the water isn’t hot enough, the valve mixes it with cold water to give the boiler a big enough temperature difference. If the temperature is above 48 degrees C, water goes straight to the tap. If water is between 48 and 28 degrees C, the solar pre-warmed water is mixed down to 28 degrees C. You still need a tank for this system with a dedicated electric element to sterilize the tank once a week. Flow rates would still be limited to the Combi boiler flow rate so there are limited benefits over a dedicated unvented hot water cylinder.

I have a system gas boiler and hot water tank, what would I need to be compatible with a solar hot water system?

A hot water cylinder with solar thermal heating and backup gas boiler heating needs to be the twin coil type. This means that both solar thermal system and gas boiler have a dedicated coil within the tank for indirect water heating.

200L: 600mm diameter x 1500mm height

250L: 600mm diameter x 1750mm height

300L: 650mm diameter x 1750mm height

I have a direct electric hot water heating system. How do I install solar thermal water heating?

With a direct electric hot water heating system, a dedicated coil within the hot water tank would be needed for the solar thermal system. It is likely that you will therefore need to get a new hot water cylinder. It may be necessary to install a timer for the back-up immersion heater if not already present so that the electric backup can be programmed to come on at the end of the day.


Explanation of solar thermal components

Solar Thermal FAQsSolar collector

A flat plate collector is specially designed to trap heat from the sun. A special black absorber surface collects solar irradiation and high quality glazing reduces heat loss from inside the collector. Even when it is 17 degrees outside the collector can reach 40-50 degrees.

Solar Thermal FAQsHot water cylinder

Heat from the solar collector warms the bottom of the cylinder next to the cold inlet. A back-up heating system e.g gas boiler increase the temperature of the cylinder to 60 degrees every evening when there is not sufficient heat from the solar thermal system alone

Solar Thermal FAQsController

The Controller measures the temperatures in the collector and storage cylinder. When the collector temperature is hotter than the bottom of the hot water cylinder, the controller switches on the pump which circulates the heat from the collector into the tank.


Solar Thermal FAQsHeat transfer fluid

An anti-freeze heat transfer fluid containing 40% mixture of propylene glycol and water is used to protect the external collector pipework down to -20 degrees C in the winter.



Solar Thermal FAQsPumpstation

The Resol Pumpstation contains a flow meter to measure the system flow rate, temperature dials to show how hot the heat transfer fluid is going to and from the collector, a high quality Wilo circulation pump, flushing and filling points to allow a flushing pump to purge air out of the system and the Resol controller.



Solar Thermal FAQsExpansion vessel

As the heating fluid gets hotter it expands and an expansion vessel with an air membrane inside helps to keep the system pressure stable.



Solar Thermal FAQsDischarge vessel

Sometimes when the system has not been adequately purged of air the system pressure can get very high and will escape out of the pressure relief valve. This fluid is captured in the discharge vessel so that the homeowner can see evidence of the problem and report this to their solar thermal engineer.


Kaspar Bradshaw, Project Engineer, Enhabit

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